1. Field of the Invention
The present invention relates to a small and thin slot antenna that can be incorporated into a consumer device, and particularly to a slot antenna that is incorporated into a consumer device and has excellent characteristics in a UWB service band.
2. Description of the Related Art
More specifically, the present invention relates to a slot antenna that is incorporated into a consumer device, has stable characteristics without being affected by an electromagnetic wave from a peripheral circuit within the device or a reflector around the device, and eliminates effects on a peripheral high-frequency circuit, and particularly to a slot antenna that can be used at a very short distance to another identical slot antenna when performing transmission and reception to and from the other identical slot antenna, and can be used in a wide communication area.
Attention has recently been directed to a radio communication system using a very wide frequency band of 3.1 GHz to 10.6 GHz, which system is referred to as “Ultra WideBand (UWB) communications”. While the UWB communications assume a PAN (Personal Area Network) with a communication distance of about 10 m, the UWB communications are expected to be put to practical use as a radio communication system for achieving a short-distance ultrahigh-speed transmission at a transmission speed of about 100 Mbps (see Mar. 11, 2002, issue of Nikkei Electronics “Advent of Revolutionary Technology-Ultra Wideband” pp.55 to 66, for example).
In IEEE802.15.3, for example, a data transmission system for transmitting data having a packet structure including a preamble is devised as an access control system for the UWB communications. Intel Corporation of the U.S. is considering a wireless version of USB (Universal Serial Bus) spread as a general-purpose interface for personal computers as a UWB application.
While UWB is a radio communication system for short distances because of its transmission power, UWB enables high-speed radio transmission. Therefore UWB can be applied as a system for consumer use when a mobile digital device such as a digital camera, a music reproducing device or the like is connected to a television set or a personal computer at a short distance by radio, so that contents such as music, images and the like can be transmitted as data at a high speed.
Meanwhile, in consideration of the capability of data transmission exceeding 100 Mbps and ease of creation of an RF circuit, transmission systems using a UWB low band of 3.1 to 4.9 GHz without occupying the transmission band of 3.1 GHz to 10.6 GHz are being actively developed.
Recently, there appeared a mobile device incorporating a radio communication function for a purpose of exchanging data such as images, music and the like with a personal computer (see http://pcweb.mycom.co.jp/news/2002/09/03/10.html, for example). The present inventor et al. consider that a data transmission system using the UWB low band is one of effective radio communication technologies to be incorporated into this kind of small mobile device.
The present inventor et al. consider that when UWB communication technology is applied to a mobile device, an antenna for UWB transmission needs to be designed to be of a small size, as with the main body of the device. In addition, to incorporate the antenna into the device, the antenna needs to be designed such that the radio system obtains tolerable frequency characteristics by avoiding effects of an electromagnetic wave from another circuit within the device and effects of a reflector around the device. Conversely, effects of a high-frequency component of a wide band used by the UWB antenna on another high-frequency circuit within the same device need to be eliminated.
Since UWB is a short-distance high-capacity radio communication system, UWB is expected to be applied to high-speed data transmission in an ultrashort-distance area such as in an ultrahigh-speed DAN (Device Area Network) for short distances including a storage device. In such a case, the UWB antenna needs to secure both excellent reflection characteristics and excellent coupling characteristics as antenna characteristics even at an ultrashort distance of about 5 to 150 mm.
An antenna basically uses a resonance phenomenon, and a resonance frequency is determined by the length of the antenna. Hence, in UWB communication in which transmission is performed using a wide frequency band, it is difficult to produce resonance over the service frequency band (3.1 GHz to 10.6 GHz).
When UWB is applied to ultrashort-distance communication, it is difficult to obtain desired antenna characteristics because an electric field and a magnetic field behave independently in a near electromagnetic field of the length of about one wave. While this may not be the case with a narrow band, reflection characteristics need to be obtained in a wide range in UWB communication, and therefore it is more difficult to obtain desired antenna characteristics.
Further, when antennas are used at an ultrashort distance, there is reflection from a device including the antennas performing communication and reflection from the ground board of the antennas. Therefore design for obtaining desired characteristics is difficult.
To ensure high-capacity communication at 100 Mbps or more such as UWB communication or the like needs an antenna to be designed such that reflection characteristics are −10 dB or lower within a bandwidth of a defined specification. In addition, to obtain a throughput of 100 Mbps or more within the bandwidth of the UWB defined specification needs the antenna to be designed such that the coupling characteristics of the antenna do not have a sharp gain attenuation, and an overall gain is at a certain level or higher.
When decreasing size and thickness is considered, a so-called patch antenna and a slot antenna can be cited. The patch antenna is formed by arranging a radiating conductor and a ground conductor in such a manner as to be opposed to each other with an insulating material as an interposed object. The slot antenna is formed by a conductor pattern, a ground layer, and a dielectric layer interposed between the conductor pattern and the ground layer, with a slot made in the ground layer.
For example, a low-profile patch antenna having a wide band and sufficient gain is proposed in which a substance having a conductivity characteristic such that conductivity of the substance is about 0.1 or more and 10 or less is interposed between a radiating conductor and a conductor ground board, whereby the substance having the conductivity characteristic causes a proper amount of signal leakage between the conductor ground board and the radiating conductor (see Japanese Patent Laid-Open No. 2003-304115, for example).
While the slot antenna and the patch antenna are both advantageous in terms of reduction of antenna size and thickness, the slot antenna and the patch antenna do not inherently provide desired characteristics over a wide band. Thus, whichever structure is employed for an antenna, to design the antenna as an antenna for UWB needs some device for widening the band.
In general, when the slot antenna and the patch antenna have equal dimensions, the slot antenna can obtain a wider band than the patch antenna. The patch antenna has an operating band of a few percent as an antenna. Hence, the present inventor et al. consider that the slot antenna is more appropriate as an antenna for UWB to be used within a consumer device.
When the slot antenna is used as an antenna for UWB within a consumer device, and when radio communication is performed at an ultrashort distance, there are problems of effects of an electromagnetic wave from another circuit within the device and effects of a reflector around the device, effects of a high-frequency component of a wide band being used on another high-frequency circuit within the same device, and reflection from the device including the antenna performing communication and reflection from the ground board of the antenna (described above). However, there are practically no publicly known techniques regarding antennas for UWB to be incorporated into devices.